The present invention generally relates to magnetic recording of information and more particularly to a magnetic disk apparatus called hard disk drive and a surface coating provided on a magnetic disk used in such a hard disk drive.
Hard disk drives are used extensively in various computers as a large-capacity, high-speed auxiliary storage device.
A typical hard disk drive includes a rigid magnetic disk rotated at a high speed and a magnetic head held on a swing arm so as to scan the recording surface of the magnetic disk, wherein the magnetic head scans the recording surface at a high speed generally in a radial direction of the magnetic disk in response to the rotational movement of the swing arm. The magnetic disk is usually rotated at a speed of several thousand r.p.m. and the magnetic head carries out reading or writing of information in the state that the magnetic head is floated from the surface of the magnetic disk by a minute distance.
A typical hard disk drive includes a plurality of magnetic disks mounted coaxially on a common drive hub and the swing arm and the magnetic head are provided for each magnetic disk. The swing arms corresponding to the magnetic disks are mounted on a common drive shaft in the form of a unitary body, and the magnetic heads on the respective swing arms scan the surface of the corresponding magnetic disk simultaneously in response to the rotation of the drive shaft.
FIG. 1 shows the internal structure of a typical hard disk drive in a plan view, wherein the left side of the broken line shows the hard disk drive in a state wherein the upper cover is removed, while the right side of the broken line shows the construction of a magnetic disk 11 and an arm assembly 12 that cooperates with the disk 11, wherein the magnetic disk 11 and the arm assembly 12 form a part of a magnetic disk assembly 10 in which a plurality of magnetic disks are stacked with each other.
Referring to FIG. 1, each magnetic disk 11 is mounted upon a hub 11a that is driven by a motor not illustrated, and the arm assembly 12 includes a swing arm 12b held on a swing axle 12a and a magnetic head 12c provided on a free end of the arm 12b. Further, a coil 12d that forms a part of a voice coil motor 13 is provided on the arm 12b in correspondence to another free end, opposite to the first free end on which the magnetic head 12c is provided, wherein the coil 12d is wound parallel to the scanning surface of the arm 12b. Further, magnets 13a and 13b forming another part of the voice coil motor 13 are disposed above and below the coil 12d. Thereby, the arm 12 is rotated about the swing axle 12a freely in response to the energization of the coil 12d. The voice coil motor 13 is subjected to a servo control such that the magnetic head 12c carried on the arm 12b properly tracks a cylinder or track 11b that is defined on the magnetic disk 11.
FIG. 2 is a perspective view showing the internal structure of the hard disk drive of FIG. 1.
Referring to FIG. 2, the magnetic disk assembly 10 includes a plurality of magnetic disks 111, 112, . . . that are held commonly on the rotary hub 11a, and the arm assembly 12 includes a plurality of arms corresponding to the plurality of magnetic disks. Each arm 12b is held on a common rotatable member 12e that in turn is held rotatable about the swing axle 12a, and all the arms 12b are swung simultaneously in response to the rotational motion of the member 12e. Of course, the member 12e is activated in response to the energization of the voice coil motor 13. Further, the entire structure of the hard disk device is accommodated within a sealed envelope 1.
FIG. 3 shows the cross-sectional structure of the magnetic disk 11.
Referring to FIG. 3, the magnetic disk 11 is formed on a substrate 11A of Al, and the like, and includes a foundation layer 11B typically of NiP formed on the substrate 11A with a thickness of about 10 μm, a Cr layer 11C formed on the foundation layer 11B with a thickness of about 300 nm, and a magnetic film 11D of a Co alloy formed on the Cr foundation layer 11C with a thickness of about 300 nm, wherein the magnetic film 11D holds written information in the form of magnetization.
Further, the surface of the magnetic film 11D thus formed on the magnetic disk 11 is protected against damages caused by a physical contact of the magnetic head 12c as in the case of head crashing event, by providing a hard carbon film 11E on the surface of the magnetic film 11D, wherein the hard carbon film 11E is formed with a thickness of about 10 nm and has a diamond-like structure. Thus, the hard carbon film 11E is called a DLC (diamond-like carbon) film.
The hard carbon film 11E thus formed is further covered by a lubricating film 11F with a thickness of about 2 nm, wherein the lubricating film 11F is typically formed of a fluorocarbon resin and is provided so as to reduce the friction between the magnetic head 12c and the magnetic disk 11. By providing the lubricating film 11F, damaging of the magnetic film 11D at the time of contact-start-stop operation of the hard disk drive, and the like, is minimized.
In the actual magnetic disk, the layered structure of FIG. 3 is formed not only on the topside of the magnetic disk 11 but also on the bottom side thereof.
Conventionally, it is practiced to form the DLC film 11E by a sputtering process that uses a graphite target. In order to achieve a high deposition rate at the time of the deposition of the DLC film 11E, it is commonly practiced to use a high-density d.c. magnetron sputtering process for the sputter deposition of the DLC film 11E. In such a conventional d.c. magnetron sputtering process of the DLC film 11E, it is further practiced to add a gas containing hydrogen such as H2 or CH4 to the sputtering gas, typically of Ar, so as to terminate the dangling bonds in the DLC film 11E. By doing so, it is possible to form the DLC film 11E in the form of an insulating film.
FIG. 4 shows the construction of a d.c. magnetron sputtering apparatus 20.
Referring to FIG. 4, the d.c. magnetron sputtering apparatus 20 includes a processing chamber 21 evacuated at an evacuation port 21A, wherein the processing chamber 21 accommodates therein a substrate 22 to be processed.
In the processing chamber 21, there is provided a graphite target 23A and a graphite target 23B such that the graphite target 23A faces a top surface of the substrate 22 and the graphite target 23B cases a bottom surface of the substrate 22.
Above the processing chamber 21, there is provided a magnet 25A centrally to the graphite target 23A in the state that the N-pole of the magnet 25A faces the graphite target 23A. Further, an annular magnet 26A is provided at the top part of the processing chamber 21 in the state that the S-pole of the magnetic 26A faces the graphite target 23A, wherein the magnet 26A is provided so as to surround the central magnet 25A.
Similarly, a magnet 25B is provided on the bottom part of the processing chamber 21 in the state that the N-pole of the magnet 25B faces the graphite target 23B. Further, an annular magnet 26B is provided around the central magnet 25B with such an orientation that the S-pole of the magnet 26B faces the graphite target 23B.
The processing chamber 21 is supplied with a mixed gas of Ar and CH4 via a mass-flow controller 24A and a line 24B, and a plasma 28A is formed in the processing chamber 21 adjacent to the target 23A by supplying a d.c. power from a d.c. power source 27A to the target 23A. Similarly, a plasma 28B is formed in the processing chamber 21 adjacent to the target 23B by supplying a d.c. power from a d.c. power source 27B to the target 23B.
The plasma 28A thus formed acts upon the surface of the graphite target 23A and the sputtered C atoms are deposited on the top surface of the substrate 22 to form a DLC film corresponding to the DLC film 11E. Similarly, the plasma 28B acts upon the surface of the graphite target 23B and the sputtered C atoms are deposited on the bottom surface of the substrate 22 to form a DLC film corresponding to the DLC film 11E.
In such a d.c. magnetron sputtering apparatus, it is possible to form a ring-shaped plasma region of high plasma density in the plasma 28A by disposing the magnet 26A around the magnet 25A, wherein the sputtering process is promoted in such a high-density plasma region and the deposition of the DLC film on the top surface of the substrate 11E is facilitated. Similarly, it is possible to form a ring-shaped plasma region of high plasma density in the plasma 28B by disposing the magnet 26B around the magnet 25B, and the deposition of the DLC film on the bottom surface of the substrate 22 is facilitated.
On the other hand, the inventor of the present invention has discovered a problem in such a d.c. magnetron sputtering process of the DLC film that uses a ring-shaped high-density plasma in that deposition of a DLC film or a similar structure occurs not on the substrate 22 but on the graphite target 23A or 23B in the central part where the plasma density is low. It is believed that such a DLC film covers uniformly on the surface of the target 23A or 23B at the beginning but is removed as a result of sputtering in the ring-shaped region where the plasma density is high. Thus, in such a ring-shaped region of the target 23A or 23B, exposure of the fresh target surface is maintained.
It should be noted that the DLC film thus deposited on the central part of the target 23A or 23B is an insulating film and easily causes charge-up as a result of contact with the plasma 28A or 28B. Thereby, the DLC film tends to cause scattering in the processing chamber 21 and forms exotic fragments on the surface of the substrate 22. Thereby, the yield of the DLC film on the substrate 22 is reduced.
In the magnetic disk 11 of FIG. 3, the lubricating film 11F performs an important function of reducing friction between the magnetic disk surface and the slider surface of the magnetic head as noted before. It should be noted that the slider surface of the magnetic head makes a direct contact with the magnetic disk 11 in the contact-start-stop operation of the magnetic disk drive. In view of the fact that the magnetic disk drive is used in various environmental conditions, it is required that the lubricating film 11F functions as an effective lubrication layer over a wide range of temperature and/or moisture condition. The importance of the lubricating film 11F is increasing further in recent magnetic disk drives having increased recording density characterized by a very narrow gap between the magnetic disk surface and the magnetic head. With regard to the lubricating film 11F, reference should be made to the U.S. Pat. Nos. 3,778,308, 4,267,238 and the 4, 268, 556.
Conventionally, a perfluoro resin compound having a highly polar group such as OH group or COOH group as the endcap group has been used for the lubricating film 11F. In such a lubricating film 11F of perfluoro resin compound, the polar end group of the perfluoro-resin compound functions to cause adherence or bonding of the lubricating layer 11F on the underlying surface of the hard carbon film 11E.
On the other hand, such a conventional lubricating film 11F using conventional perfluoro resin compound encounters a problem in that, while the bottom part of the lubricating film 11F is thus bonded firmly upon the surface of the underlying hard carbon film 11E, the upper part of the lubrication film 11F is not bonded and forms a mobile component that can move freely over the surface of the hard carbon film 11E and hence the surface of the magnetic disk 11.
Thus, there is a tendency that the mobile component of the lubricating film 11F is displaced in the radial direction of the magnetic disk 11 toward the peripheral edge thereof as a result of the centrifugal force caused by the high-speed rotation of the magnetic disk. The mobile component thus displaced is accumulated at the peripheral part of the magnetic disk and increases the disk thickness of the peripheral part at the magnetic disk 11.
Thus, the existence of such a mobile component causes a serious problem particularly in recent high-recording-density magnetic disk drives in which the air gap formed between the magnetic disk surface and the magnetic head is reduced and the rotational speed of the magnetic disk is increased.
In order to reduce the proportion of the mobile component, it is necessary increase the proportion of the bonded component in the lubricating film 11F, and it has been proposed to form the lubricating film 11F immediately after the formation of the hard carbon film 11E such that the lubricating film 11F causes a reaction with the reactive surface of the hard carbon film 11E. However, the reactive surface of the hard carbon film 11E adsorbs a large amount of impurities in the air within a short time after formation of the hard carbon film 11E. Thus, such a process has been not effective for reducing the mobile component.
Further, it has been proposed to activate the surface of the hard carbon film 11E by a plasma process as disclosed in the Japanese Laid-Open Patent Publications 62-150226 and 63-2117, or by irradiating ozone ultraviolet radiation as taught in the Japanese Laid Open Publications 4-6624 and 6-301970. However, there still remains difficulty to increase the proportion of the bonded layer in the lubricating film 11F in the magnetic disk of FIG. 3.
Further, it has been proposed in the Japanese Laid-Open Patent Publication 7-262555 and 8-124142 to induce a reaction in the lubricating film 11F to cause a bonding with the underlying hard carbon film 11E by applying optical radiation to the lubricating film 11F after deposition on the hard carbon film 11E such that optically excited electrons form radicals that facilitates the bonding reaction of the lubricating film 11F upon the hard carbon film 11E.
However, such a process is effective only after optical exposure conducted for a long period of time and the throughput of production of the magnetic disk drive is deteriorated substantially. Further, there is a risk in such an optical radiation process in that the surface of the lubricating film 11F may be oxidized as a prolonged optical exposure. When such oxidation is caused, the friction at the surface of the lubricating film 11F may be increased. Further, such optical processing is conducted in the air and the chance that impurities in the air contaminate the lubricating film 11F. When such contamination is caused, there is a possibility that the impurities on the lubricating film 11F may act as nuclei of water drop condensation and the water drops act to collect further impurities in the form of condensates. Thereby, the condensates may cause the problem of increased drive torque of the magnetic disk due to increased friction and further various problems such as contamination of the magnetic head, deterioration of floating of the magnetic head, corrosion of the magnetic head, and the like.